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How do you know power is at the outdoor unit? Did you check it with a meter? Check your breaker. If there's power at the outdoor unit, then shut the power and test your contacter. If the contacter has supply power then check the low voltage (24-28 volts) that operates the contacter, and if none check the low pressure cut-out,high pressure cut-out and make sure the thermostat is calling for cool. All ok so far, then test your capacitor. I forgot to mention to check for loose, broken, and disconnected wires.

Doc, the outside compressor could be low on refrigerant gas or the condenser switch in the outside unit is faulty. One of the things that is most overlooked is a faulty wall thermostat. For now, it's a process of elimination to troubleshoot and solve the problem.

You picked the wrong picture...lol. You have a split system and that humm you hear comes from the contactor. That means the thermostat is asking the compressor which is outside to come on. If you hear the humm but nothing is working, Check the breaker and then replace the fuses in your fuse box by the condensing unit, or the outside unit if the breakers are OK. It should have a disconnect between 3 to 10 feet of the unit depending on your codes and city requirements. No matter what, if you stand at your unit and look at the conduit, it will lead to your electrical disconnect. It could be a sub-breaker or fuses. It is usually fuses because a breaker could take up to 6 sin waves to respond as where a fuse could take only 2 sin waves. Replace both fuses. As time goes on, fuses become warn from constant load changes and heat and sooner a latter, they just wear out like any other component. Change the fuses and fire your unit back up. If it pops the new breakers, you have other issues and it's time to call an HVAC guys.

It sounds like you're looking in the right place. Check your power panel for a tripped circuit breaker (maybe more than one) or if the home is older, it may have fuses. It's possible that the outdoor disconnect also has fuses but that's not as likely. You can check around the outdoor unit and see if you smell anything burnt.

You can aso turn off the unit at the thermostat, wait a few minutes and see if the unit starts back up, but check the items listed above first.

are you talking about a 'split system' here - as in a house - where you have an inside unit and an outside unit?

If so - when you say you had the 'breaker' off - are you talking about the breaker to the 'outside' unit or the breaker to the 'inside' unit?

Note: on split systems you have a breaker that controls the outside unit and you have a breaker that controls the inside unit.

Typically - the outside unit has a breaker box/fuse box outside with the unit although you will also have a breaker in the main breaker box too, which is usually located in the garage or house somewhere.

The inside unit (usually located in the garage or house somewhere) will not have a breaker box/fuse box, (will merely be plugged in to the wall) but will have a breaker - also - in the main breaker box.

The importance of these questions is because it's possible you 'shorted' your transformer (located in the inside unit) when you changed the thermostat. Although if you had the inside breaker off while you were changing it (thermostat) - I don't see how you could of did this.

In any event - do you remember seeing a spark when you were changing out the thermostat?

If so, then I'm betting you shorted out the transformer.

If that's the case - I would recommend calling a Service Tech - because changing a transformer and 'clearing the short' is sometimes a complicated process - and unless you are fairly adept mechanically and electrically - I would advise getting a professional to do this.

In any event - he/she should be able to solve your problem for only a nominal service call.

You must make sure that there is power to both the inside unit and outside unit. Is the room air blower blowing? If not you have either a plugged filter, iced up coil, the access door is not quite closed or there is no power to the unit. Is the outside unit fan turning? If not, you either have no call from the thermostat, the fan is broken or the unit has no power to it. I would turn off the breaker to the inside unit and switch the breaker near the outside unit to off and let them sit idle for 5 minutes. Switch both breakers back on and chect to see if the outside unit turns on. Then check the indoor unit and see if this starts blowing. Let us know what you find.

Sounds like you either blew a transformer, have a bad circuit breaker or blew the main fuse in the outside disconnect. If the indoor unit is still blowing air (no matter what temperature) start looking at the power supply to the outdoor unit. From the circuit breaker, the power will go to a small box located within a few feet from the outside unit. This box will have either a lever on the side or you will be able to open the box and pull out the fuses. From this box the power goes to the condensor. The first thing you should do is to turn off the breaker to the outside unit. Flip it back on and if you have a call for cooling, after about 3 minutes the outdoor unit should start. If it does not, shut off power again to the unit by switching off the breaker, go outside and pull out the fuses in the disconnect box. Using a multi meter, check for continuity thru the fuses. If you have continuity, call your technician. If one or both fuses show no sign of continuity, replace the fuse(s).
Make sure that the thermostat is working and sending a signal to the indoor air handler. You may be able to check this by turning the fan switch to "Fan" and see if the blower turns on. You may just have a bad thermostst.
Caution should be used anytime you are near electrical components. If you do not have the skill-set required to test electrical equipment, leave it to a proffessional.